Payment to join navy's divers (4)
I believe the answer is:
subs
'navy's divers' is the definition.
Although both the answer and definition are plural nouns, I cannot see how they can define each other.
'payment to join' is the wordplay.
'payment' becomes 'sub' (I have seen 'Advance payment ' mean 'sub' so perhaps 'payment' could also mean 'sub').
'to join' becomes 's' (I am not sure about this - if you are sure you should believe this answer much more).
'sub'+'s'='SUBS'
Can you help me to learn more?
(Other definitions for subs that I've seen before include "Advance payments" , "Players in reserve" , "Reserves at football (sl.)" , "vehicles" , "Club members' payments" .)